Active regions in a silicon semiconductor substrate are conventionally separated by growing silicon dioxide dielectric in selected regions using the substrate silicon and an oxidizing environment. The active regions are masked with a patterned silicon nitride layer to prevent oxidation. Nevertheless, the silicon dioxide growths extend beneath the edges of the masking silicon nitride layer to form undesired protrusions of silicon dioxide commonly known as the "bird's beak". Investigations directed toward the reduction of the bird's beak have identified that a major contributor to the formation is the thin silicon dioxide pad or buffer layer, which layer is conventionally formed beneath the silicon nitride masking layer to avoid dislocation defect type damage otherwise induced by the thermal coefficient of expansion mismatch between any thick silicon nitride mask and the monocrystalline silicon substrate.
Various techniques for suppressing the lateral movement of oxygen species through the pad layer underneath the silicon nitride mask have been evaluated, with varying degrees of success. For instance, the use rapid thermal nitridation to convert the pad/buffer silicon dioxide (oxide) layer, or merely the native oxide layer, to a graded oxynitride has exhibited relatively good results and is described in co-pending U.S. patent application Ser. No. 07/110,245 (NCR Docket 3967), assigned to the assignee of the present application. Another technique, involving the use of a thin sealing silicon nitride (nitride) layer deposited directly over the native oxide or the nitrided native oxide, was proposed in the article entitled "Selective Oxidation Technologies for High Density MOS" by authors Hui et al., as appeared in the IEEE Electron Device Letters of October 1981. Extensions of the sealed interface localized oxidation (SILO) concept first proposed in the article by Hui et al. are developed in U.S. Pat. Nos. 4,472,459 and 4,551,910. The former patent proposes the deposition of the thin nitride layer on the substrate silicon, without the express formation of an intermediate pad/buffer oxide layer. The implications of the native oxide layer are not addressed. A similar absence of appreciation for the effects of the native oxide is apparent in the teachings of the latter identified patent, where the process expressly prescribes the formation of the nitride layer over the native oxide layer.
Sequential processing in the context of semiconductor fabrication is taught in U.S. Pat. No. 4,438,157, wherein there are described processes for in situ deposition of multiple dielectric layers during the formation of nonvolatile memory devices.
There remains a need for further reducing the bird's beak effect during field oxide growth in silicon semiconductor substrates. The importance of eliminating or substantially suppressing the bird's beak protrusion is evidenced by the major resources being expended to evaluate various trench isolation structures and processes as documented in recent patent and technical literature.